Theo C. M. Bakker

According to the mate attractiveness hypothesis of biased progeny sex-ratio, sexually attractive males should produce relatively more sons. We tested this hypothesis with three-spined sticklebacks Gasterosteus aculeatus , a fish species with external fertilisation, in which the intensity of red breeding coloration is a sexually selected trait. We used reflecto-spectrophotometry and visual modelling to assess red breeding coloration from a stickleback’s point of view. More intensely red-coloured males produced relatively more sons as assessed with molecular sexing three weeks after hatching of 34 clutches from different parents. Confounding factors like mortality rate and maternal and paternal effects other than male coloration could largely be ruled out. We discuss potential mechanisms for the role of fathers in sex-ratio allocation.

We studied phenotypic and genetic adaptation of the visual system of three-spined sticklebacks, Gasterosteus aculeatus, from North Uist, Scotland. We quantified differences in opsin gene expression of the four cone opsin genes among wild-caught fish from three lakes with clear and from three with tea-stained water and their offspring that were raised in clear water. In addition, visual sensitivity of wild-caught fish was modelled from opsin expression levels. Wild-caught fish from tea-stained waters had a lower SWS1 proportional expression than fish from clear waters, a difference that tended to be maintained in lab-bred fish. Compared to lab-bred fish, wild-caught fish had a higher SWS1 but lower SWS2 proportional expression independent of water clarity. For RH2 and LWS there were significant interactions between generation and water clarity. Reproductively mature fish had a higher LWS but lower proportional expression of RH2 than non-reproductive fish. Sex did not have a significant effect on expression. There was a significant positive association and, depending on chromophore ratio, a distinct match between the centre wavelengths, used as a proxy for spectral distribution, of ambient habitat light and spectral sensitivity indicating that the visual system of sticklebacks is tuned to their local light environment, suggesting adaptation.

Different environmental conditions may lead to diverse morphological, behavioral, and physiological adaptations of different populations of the same species. Lighting conditions, for example, vary vastly especially between aquatic habitats, and have been shown to elicit adaptations. The availability of short-wave ultraviolet (UV) light is especially fluctuating, as UV wavelengths are attenuated strongly depending on water properties. The island of North Uist, Scotland, comprises 2 differential habitat types, tea-stained and clear-water lakes, varying considerably in UV transmission. In previous studies, wild-caught 3-spined stickleback Gasterosteus aculeatus populations (3 populations of each habitat type) were tested with respect to their shoaling and mate preferences for fish viewed under UV-present and UV-absent conditions. The results revealed a habitat-dependent preference of UV cues during shoal choice (tea-stained populations: preference for UV-absent condition in tea-stained water; clear-water populations: no preference in clear-water) but an overall preference for UV-present conditions during mate choice. To assess genetic influences on these behavioral patterns, similar experiments were conducted with lab-bred F1-generations of the same stickleback populations that were raised in a common environment (i.e. standardized clear-water conditions). Offspring of sticklebacks from tea-stained lakes tended to prefer shoals viewed under UV-absent conditions (only in tea-stained water), while sticklebacks from clear-water lakes showed a significant preference for the shoal viewed under UV-present conditions in clear-water but not in tea-stained water. Mate-preference experiments demonstrated that females from the tea-stained lakes significantly preferred and females from the clear-water lakes preferred by trend the male viewed under UV-present conditions in the clear-water treatment. The results for both shoaling- and mate-preference tests were largely similar for wild-caught and lab-bred sticklebacks, thus hinting at a genetic basis for the preference patterns.

Ultraviolet (UV) A signals (320–400 nm) are important in mate choice in numerous species. The sensitivity for UV signals is not only assumed to be costly, but also expected to be a function of the prevailing ecological conditions. Generally, those signals are favored by selection that efficiently reach the receiver. A decisive factor for color signaling is the lighting environment, especially in aquatic habitats, as the visibility of signals, and thus costs and benefits, are instantaneously influenced by it. Although ecological aspects of color signal evolution are relatively well-studied, there is little data on specific effects of environmental UV-light conditions on signaling at these shorter wavelengths. We studied wild-caught gravid female 3-spined sticklebacks Gasterosteus aculeatus of 2 photic habitat types (tea-stained and clear-water lakes), possessing great variation in their UV transmission. In 2 treatments, tea-stained and clear-water, preferences for males viewed under UV-present (UV+) and UV-absent (UV–) conditions were tested. A preference for males under UV+ conditions was found for females from both habitat types, thus stressing the significance of UV signals in stickleback’s mate choice decisions. However, females from both habitat types showed the most pronounced preferences for males under UV+ conditions under clear-water test conditions. Moreover, reflectance measurements revealed that the carotenoid-based orange-red breeding coloration in wild-caught males of both habitat types differed significantly in color intensity (higher in clear-water males) and hue (more red shifted in clear-water males) while no significant differences in UV coloration were found. The differential reflection patterns in longer wavelengths suggest that sticklebacks of both habitat types have adapted to the respective water conditions. Adaptations of UV signals in a sexual context to ambient light conditions in both behavior and coloration seem less evident.

Fish that perform paternal care may increase their fitness by choosing nest sites that enhance survival and development of embryos. We studied nest-site choice with respect to dissolved oxygen concentration and water temperature in males of the three-spined stickleback (Gasterosteus aculeatus), a small fish species with exclusive male parental care that usually breeds in the littoral zone of freshwaters of the Northern hemisphere. Fathers oxygenate the embryos by fanning movements of their pectoral fins. We expected choice for conditions at potential nest sites that would benefit offspring development, i.e., higher temperature and higher dissolved oxygen concentration. In the laboratory, we offered males a choice between two potential nest sites that differed in dissolved oxygen concentration or water temperature. Males preferred to build a nest at sites with a higher dissolved oxygen level or higher temperature and thus chose sites that would promote embryo development.